I created the L-Phenylalanine based Evans auxiliary bearing a benzyl group with help of a molecular model set and got into some questions. The structural formula will do:

As I suppose a low rotational barrier, the phenyl group can be aligned off (a) and towards (b) the alpha position of the N-propionyl group. I suppose maximum sterical repulsion for attacking electrophiles for (b).

The phenylalanine and valine based auxiliaries (Bn or i-Pr) are the most commonly involved types of Evans auxiliaries. But why do I get good stereoselectivities for this Bn auxiliary? There are no corresponding carbon-carbon rotational freedoms in the valine species.

You have to imagine the nucleophile attacking from "behind" your screen. The phenyl does have some freedom to move, but it's sitting in front of your screen and thus blocks access from the front, not the back. Assuming the Evans auxiliary is planar, of course.
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CHMSep 23 '12 at 17:43

1 Answer
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The comment left by CHM is very helpful. You need to consider what this molecule actually looks like in 3D. That may help your understanding greatly, but to be honest there is considerably more going on here than you would expect.

A couple pieces of information would be very helpful for clarifying this question. The first is what type of a metal are you using for this synthesis step. The second is what is the outcome. Believe it or not, based on those two pieces of information the mechanisms for this reaction can be explained entirely differently.

Since the answer basically breaks down into two possibilities I will attempt to explain both. One of the most helpful articles I came across in my search was entitled "Transition State Models for Probing Stereoinduction in Evans Chiral Auxiliary-Based Asymmetric Aldol Reactions" J. Am. Chem. Soc., 2010, 132 (35), pp 12319–12330.

In this article they go into depth about how the electronic and steric effects balance out to produce the almost complete stereocontrol throughout the course of this reaction. The scheme below is what I find the most helpful:

Note how the selectivity is varied based on whether the auxiliary is chelating to the titanium or not. This is why the metal lewis acid used in these reactions is important.

One final note not shown here is the rational behind the non chelating setup. The opposing dipoles on "y" and aldehyde oxygen in the Zimmerman-Traxler model help to hold the six member ring in place while the aldol reaction occurs.

The scheme shown is incredibly useful for visualizing this transformation and I would like to compliment the authors for their efforts in this publication.